Epoxy resins are monomers or prepolymers that further react with curing agents to yield high-performance thermosetting plastics. They have gained wide acceptance in protective coatings and electrical and structural applications because of their exceptional combination of properties such as toughness, adhesion, chemical resistance, and superior electrical properties.
Epoxy resins are characterized by the presence of a 3-member cyclic ether group commonly referred to as an epoxy, 1,2-epoxide, or oxirane group. The epoxy resins are commonly cured, or caused to harden, by the addition of a curing or hardening agent. Curing agents commonly used to convert epoxy resins to thermosets include anhydrides, amines, polyamides, Lewis acids, and others. Many curing agents begin to react immediately, or after a short period of time, even at room temperature or lower temperatures. This requires that the epoxy resin and the curing agent be stored separately and only mixed immediately before use. This is extremely inconvenient in some cases and also leads to waste if part or all of the mixture cannot be used in time.
While various attempts have been made to prolong the cure time or "pot life" of curable epoxy resin compositions, they have met with limited success. The preferred curing agents, Lewis acids, cannot be used where appreciable pot life of the composition is required. Because of the rapidity with which the Lewis acid brings about curing of the epoxy resin compositions, various attempts have been made to modify the Lewis acid in order to prolong the cure time or pot life of the composition. One method of accomplishing this result has called for the formation of a complex of the Lewis acid with a nucleophilic reagent such as an ether, alcohol, amine or the like. It has been generally found, however, that any increase in pot life of epoxy compositions containing the modified version of the Lewis acid results in a corresponding sacrifice of cure time and/or physical characteristics of the resultant cured resin.
Various references disclosing the preparation and/or use of curing agents for epoxy resins and/or other curable compositions are discussed below.
Harris (U.S. Pat. No. 3,542,828) discloses hexafluoroantimonate amine catalysts useful in cationic polymerization of vinyl ether. Harris reacts fluoroantimonic acid or its alkaline salt with amines such as aniline, diphenylamine, triphenylamine, methylamine, diethylamine, trimethylamine, nonylamine, trichlorophenylamine, tritolylamine and the like to produce his catalyst. Experimentation has shown that the Harris catalyst does not appreciably increase the pot life of epoxy resin compositions to produce a room temperature latent composition which cures rapidly with heating. In fact, some epoxy resin compositions catalyzed with the Harris catalyst have been found to cure rapidly as low as 0.degree. C.
White et al (U.S. Pat. No. 3,565,861) discloses amine complexes of phosphorous pentafluoride, arsenic pentafluoride, or antimony pentafluoride with a primary acylic aliphatic amine to provide latent curing agents for epoxy resins. This patent points out that amines having the NH.sub.2 group attached directly to an unsubstituted aromatic ring form complexes which function as rapid curing agents but are not sufficiently stable to be latent curing agents.
Udding et al (U.S. Pat. No. 3,879,312) discloses a process for the preparation of polyester polyols by cationic polymerization of organic cyclic ethers using hydroxyl ammonium hexafluoroantimonate catalysts. Such catalysts have been found not to provide an appreciably long pot life for epoxy resin compositions and have been found to cause such epoxy compositions to cure at relatively low temperatures, even as low as 36.degree. C.
Buck et al (U.K. Pat. Spec. No. 963,058) discloses room temperature storable mixtures of epoxy resin and a curing agent provided by the amine salt of hydrofluoroboric acid. Any amine appears to be suitable including primary, secondary, tertiary aliphatic or aromatic amine, heterocyclicamine, or alicyclic amine, with or without nuclear or side-chain substituents, although the aromatic amines are designated as preferred, because of curing temperature considerations.
While certain of the modified catalysts or curing agents known in the prior art have been somewhat successful in prolonging the cure time or pot life of a curable epoxy composition, few are known to applicant to provide a significant increase in pot life which, for example, permits storage of the mixture of epoxy resin composition and curing agent for a matter of days at room temperature yet provides rapid cure at an elevated curing temperature.